1. Field of the Invention
The present invention relates generally to a speech recognition system for an automotive vehicle, and more particularly to a speech recognition system by which driver's spoken instructions can reliably be recorded or recognized even as noise fluctuates within the passenger compartment.
2. Description of the Prior Art
There is a well-known speech recognizer which can activate various actuators in response to human spoken instructions. When this speech recognizer is mounted on a vehicle, the headlight, for instance, can be turned on or off in response to spoken instructions such as "Headlight on" or "Headlight off". Such a speech recognizer usually can recognize various spoken instructions in order to control various actuators; however, there are some problems involved in applying this system to an automotive vehicle.
Usually, the speech recognizer is used in a relatively quiet environment; however, the speech recognizer system for an automotive vehicle is usually used within a relatively noisy passenger compartment subjected to wide changes in noise level. Therefore, one of the major problems is how to cope with erroneous spoken phrase recognitions caused by fluctuating engine noise within the passenger compartment.
In conventional systems a spoken instruction is distinguished from noise by providing a voice detector in the speech recognizer. The start and the end of a spoken instruction are determined by the recognizer respectively detecting whether the magnitude of a spoken instruction signal exceeds a predetermined reference threshold voltage level for a predetermined period of time and whether the magnitude of the spoken instruction signal drops below the predetermined reference threshold voltage level for another predetermined period of time.
In the prior-art speech recognizer, however, since the reference threshold voltage level is fixed, when noise level exceeds the reference threshold voltage for a long time, for instance when the vehicle is running within a tunnel, the voice detector can erroneously consider this state to represent the beginning of a spoken instruction. In other words, the prior-art speech recognizer is prone to erroneous recognition due to intense noise within the passenger compartment.
A driver has a tendency to utter a spoken instruction in a loud voice in a noisy place but in a low voice in a quiet place. In the prior-art speech recognizer, the reference threshold voltage level in recognition mode is fixed at a relatively high level for preventing erroneous recognition due to noise and the threshold level in recording mode is fixed at almost the same high level as in recognition mode. To enable the prior art system to operate properly the driver should utter a spoken instruction loudly in recording mode. However, this is contrary to the driver's inclination to speak quietly because the automobile is usually in a quiet environment while instructions are being recorded. When a spoken instruction is uttered into the microphone in a relatively loud voice, correspoding digital time-series matrix-pattern data (explained later) tend to be distorted. This results in a difference between the recording and recognition mode patterns, making it difficult to reliably recognize a spoken instruction.
A more detailed description of a typical prior-art speech recognizer and a typical prior-art voice detector will be made with reference to the attached drawings in conjunction with the present invention under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.